Offner-type imaging spectrometer

Abstract

The present invention relates to an imaging spectrometer (100) of the Offner type comprising:- an entrance slit (10) having a geometric center (11), a height (13) and a width (15), the entrance slit (10) also having a longitudinal axis (16) passing through the geometric center (11) and being oriented parallel to the height (13) of the entrance slit; - a reflective optical device (20) and a convex diffraction grating (30) in reflection arranged in a concentric configuration, - an imaging detector (540) configured to form an image of the entrance slit (10), characterized in that a straight line (60) joining the geometric optical center (31) of the diffraction grating (30) and the geometric center (11) of the entrance slit (10) forms an angle (61) with the longitudinal axis (16) of the entrance slit (10), said angle (61) being between 50 degrees and 70 degrees. Figure for abstract: Fig. 1

Description

Title of the invention: Offner-type imaging spectrometer Technical field of the invention

[0001] The present invention relates to the field of imaging spectrometers.

[0002] It relates in particular to an Offner type imaging spectrometer. State of the art

[0003] Offner-type imaging spectrometers are known, comprising an entrance slit, a diffraction grating and a reflecting optical device in a concentric configuration and an imaging detector. In these spectrometers, the slit, the imaging detector and grooves of the diffraction grating are aligned in space along a spatial axis. These spectrometers are functional since they make it possible to record on the detector a spectrum corresponding to an image of the entrance slit. In particular, it is recognized that this configuration makes it possible to obtain a spectrum of good optical quality, that is to say with low optical aberrations. It is known that this configuration makes it possible to obtain good spatial resolution in a plane of symmetry. However, in these imaging spectrometers, the spectral resolution is limited, especially when such a spectrometer has a large aperture.In this case, the best spectral resolution is obtained in the plane of symmetry. Therefore, the spatial and spectral resolution in these imaging spectrometers is significantly degraded when moving away from the plane of symmetry. Furthermore, in these imaging spectrometers, the numerical aperture of the spectrometer is low and the range of the spectrum obtained is limited. Finally, such spectrometers are bulky, leading to cost issues.

[0004] Offner-type imaging spectrometers are also known in which the slit and the diffraction grating are misaligned at an angle of 45°. Such spectrometers are functional because they make it possible to obtain a spectrum with better spectral and spatial resolution. However, this resolution is limited and is not effective over the entire height of the slit. In addition, these spectrometers are bulky.

[0005] The invention aims to resolve at least one of the aforementioned drawbacks. Presentation of the invention

[0006] In this context, the present invention proposes an Offner type imaging spectrometer comprising: - an input slit configured to transmit an incident light beam comprising wavelengths, said input slit having a geometric center, a height defined along an axis and a width, said value of the height of the inlet slot being greater than or equal to the value of the width of the inlet slot, the inlet slot also having a longitudinal axis passing through the geometric center and being oriented parallel to the axis defining the height of the inlet slot; - a reflective optical device and a convex diffraction grating in reflection arranged in a concentric configuration, i) the reflective optical device having a center of curvature and being configured to reflect the incident light beam and direct it towards the diffraction grating; (ii) the diffraction grating having a geometric optical center on the surface, the diffraction grating comprising grooves oriented parallel to the longitudinal axis of the entrance slit; said diffraction grating being arranged to spectrally diffract the incident light beam reflected on the reflecting optical device, iii) the reflective optical device being configured to reflect the light beam spectrally diffracted by the diffraction grating; - an imaging detector configured to record the light beam diffracted by said diffraction grating and reflected by the reflective optical device, the imaging spectrometer being configured to form an image of the entrance slit on the imaging detector.

[0007] The imaging spectrometer has a straight line joining the geometric optical center of the diffraction grating and the geometric center of the slit forming an angle with the longitudinal axis of the entrance slit, said angle being greater than 0 degrees and less than 45 degrees or greater than 45 degrees and less than 90 degrees. Preferably, said angle is between 50 degrees and 70 degrees. The accuracy of this angle is linked to the accuracy of the opto-mechanical device supporting the optical elements and is of the order of one arc minute or better.

[0008] Thus, in the imaging spectrometer according to the invention, the slit and the diffraction grating are misaligned, which makes it possible to obtain an imaging spectrometer with a large numerical aperture while retaining the optical quality of this imaging spectrometer. Thanks to the arrangement between the slit, the diffraction grating and the imaging detector, it is possible to obtain a spectrum with a wide spatial extent and highly spectrally and spatially resolved over the entire height of the slit. As a result, such an arrangement makes it possible to obtain better spectral and spatial resolution depending on the height of the slit.

[0009] Such an arrangement also makes it possible to obtain a more compact imaging spectrometer than state-of-the-art imaging spectrometers, such as so-called in-plane imaging spectrometers, facilitating the use or installation of such an imaging spectrometer in pre-existing devices.

[0010] In particular, here, the angle range is selected to position the diffraction grating relative to the entrance slit in order to obtain an imaging spectrometer having a large numerical aperture while preserving its optical performance. Such an arrangement makes it possible to obtain a better compromise between spectral and spatial extents and therefore improve the spectral resolution and the spatial resolution in the field compared to state-of-the-art spectrometers. Thus, the imaging spectrometer according to the invention makes it possible to provide a spectrum with better resolution over the slit height and a significant spatial and spectral extent.

[0011] In the present invention, the geometric center of an object is understood to mean the average position of the points forming the area of this object. Thus, for a two-dimensional or three-dimensional object, this corresponds in particular to the barycenter of this object obtained with weighting coefficients all equal to 1. To determine the geometric center, it is possible, for example, to calculate the point positioned at the arithmetic mean of the coordinates of each point of this object weighted with a weighting factor equal to 1.

[0012] Other non-limiting and advantageous characteristics of the system according to the invention, taken individually or in all technically possible combinations, are the following:

[0013] In one embodiment, the angle is between 45 degrees (exclusive) and 70 degrees, preferably between 50 degrees and 70 degrees. Such an arrangement makes it possible to further improve the above-described performance of the imaging spectrometer. Here, thanks to such an angle selection, the imaging spectrometer is more compact and more spectrally and spatially resolved.

[0014] In one embodiment, the angle is between 50 degrees and 60 degrees. Such an arrangement makes it possible to further improve the above-described performance of the imaging spectrometer. In particular, this configuration makes it possible to obtain the best compromise in terms of compactness of the imaging spectrometer and spatial and spectral resolution.

[0015] In one embodiment, the angle is between 15 degrees and 40 degrees. Such an arrangement allows for improved spectral extent and resolution while maintaining a maximum spatial extent close to the 0 degree on-axis configuration.

[0016] In one embodiment, the angle is between 20 degrees and 25 degrees. Such an arrangement makes it possible to further improve the above-described performance of the spectrometer compared to the in-plane configuration.

[0017] In one embodiment, the diffraction grating is arranged to diffract the incident light beam reflected by said reflecting device according to a diffraction order equal to 1 in absolute value and the imaging detector is arranged to record the diffracted light beam in the diffraction order equal to 1 in absolute value.

[0018] In one embodiment, the imaging detector comprises a recording surface having a first spatial dimension and a second spatial dimension perpendicular to the first spatial dimension, said first spatial dimension of the recording surface being oriented parallel to the longitudinal axis and being adapted to the height of the entrance slit.

[0019] In one embodiment, the detector is configured to record a spectrum of the diffracted light beam defined between wavelengths of 190 nm to 900 nm and having a spatial extent of between 28 mm and 30 mm.

[0020] In one embodiment, the imaging spectrometer further comprises a filter operating in transmission arranged upstream of the detector and configured to select a single diffraction order. Thus, all other diffraction orders are filtered (i.e. stopped) by the filter, which makes it possible to improve the spectral selectivity of the imaging spectrometer.

[0021] In one embodiment, the filter has a central axis normal to a plane of the filter and passing through a geometric center of the filter, said central axis of the filter being inclined relative to a normal to a plane of the imaging detector at an angle of inclination.

[0022] In this embodiment, said tilt angle may be between 5 degrees and 50 degrees. Such an arrangement makes it possible to limit parasitic reflections in the imaging spectrometer.

[0023] In one embodiment, the reflective optical device is arranged to correct distortion aberrations on the imaging detector.

[0024] In one embodiment, the reflective optical device comprises a concave mirror having an optical axis aligned with an optical axis of the diffraction grating.

[0025] In one embodiment, the reflective optical device comprises two concave mirrors, respectively named first mirror and second mirror, the first mirror being configured to reflect the incident light beam towards the diffraction grating and the second mirror being configured to reflect the diffracted light beam towards the imaging detector, at least one of the two mirrors among the first mirror and the second mirror being arranged concentrically with the diffraction grating.

[0026] In this embodiment, the first mirror and the second mirror may have different radii of curvature or identical radii of curvature.

[0027] In one embodiment, the entrance slot is formed by a rectangular opening or the entrance slot is formed by a plurality of optical fibers arranged along the longitudinal axis of the input slit and each having an optical axis, the optical axes of said optical fibers being parallel to each other.

[0028] In one embodiment, at least a portion of the reflecting planes have a curved shape along a radius of curvature. In this embodiment, the radii of curvature may be the same or different.

[0029] In one embodiment, the grooves are spaced apart by a constant distance. In other words, each groove is spaced from an adjacent groove by the same distance.

[0030] In one embodiment, the grooves are spaced apart by a variable distance. In other words, each groove is spaced apart from an adjacent groove by a distance, said distance varying for the grooves. Such a configuration makes it possible to limit field aberrations such as distortion. The performance of the imaging spectrometer is therefore improved.

[0031] In one embodiment, the imaging spectrometer comprises means for adjusting at least one inclination of the detector.

[0032] In this embodiment, the inclination corresponds to a rotation of the detector along at least one spatial direction or axis, preferably along at least two or even three spatial directions.

[0033] Of course, the various features, variants and embodiments of the invention may be combined with each other in various combinations provided that they are not incompatible or mutually exclusive. Brief description of the drawings

[0034] Furthermore, various other characteristics of the invention emerge from the appended description given with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

[0035] [Fig. 1] is a top view of a first embodiment of a spectrometer imager according to the present disclosure;

[0036] [Fig.2] is a schematic representation in a view plan of an arrangement of a slit and a diffraction grating of the imaging spectrometer according to the first embodiment;

[0037] [Fig.3] is a three-dimensional schematic representation of the spectrometer imager according to the first embodiment,

[0038] [Fig.4] is a schematic representation according to a profile view of a network of diffraction included in the imaging spectrometer according to the first embodiment, said diffraction grating being projected in a facial view plane perpendicular to the optical axis of the diffraction grating,

[0039] [Fig.5] is a representation of a spectrum recorded by an imaging detector included in the imaging spectrometer according to the first embodiment.

[0040] [Fig.6] is an enlarged view of the positioning of a filter relative to an imaging detector included in the imaging spectrometer,

[0041] [Fig.7] is a representation in a two-dimensional view plane of a second embodiment of an imaging spectrometer according to the present disclosure,

[0042] [Fig.8] is a three-dimensional representation of the imaging spectrometer according to the second embodiment,

[0043] [Fig.9] is another schematic representation of an arrangement of a bundle of optical fibers in the plane of the entrance slit and the diffraction grating of the imaging spectrometer according to the first or second embodiment.

[0044] It should be noted that in these figures the structural and / or functional elements common to the different variants may have the same references. Detailed description

[0045] A first embodiment of an imaging spectrometer 100 according to the present disclosure will be described with the aid of [Fig.1], [Fig.2], [Fig.3], [Fig.4], [Fig.5], and / or [Fig.6] or [Fig.9].

[0046] As illustrated in [Fig.l], the imaging spectrometer 100 comprises an entrance slit 10, a reflective optical device 20, a diffraction grating 30, an imaging detector 40 and optionally a filter 50.

[0047] In the present disclosure, the entrance slit 10 is configured to transmit an incident light beam 1. Typically, the incident light beam 1 used in the imaging spectrometer 100 is a polychromatic light beam comprising, for example, a spectrum extending between the wavelengths of 180 nm to 900 nm.

[0048] As illustrated in [Fig.2] and [Fig.3], the inlet slot 10 is formed by a rectangular opening 17 comprising a geometric center 11, a height 13 and a width 15.

[0049] In a variant of the first embodiment and the second embodiment, the inlet slot 10 may have an opening 17 of any shape, for example square, circular, parallelepipedal, or any shape, or of the shape illustrated in [Fig.9] which will be explained below.

[0050] In the present disclosure, the term geometric center is understood to mean the barycenter of the area defined by the inlet slit 10 corresponding here to the area defined by the opening 17 delimited by the perimeter of the opening 17 of the inlet slit 10. This geometric center 11 may, for example, correspond to a point positioned at the arithmetic mean of the coordinates of the points forming the opening 17. Here, as the opening 17 of the inlet slot 10 is rectangular in shape, this point also corresponds to the point of intersection of the diagonals of the opening 17 of the inlet slot 10.

[0051] The geometric center 11 of the entrance slit 10 defines the origin of a spatial reference frame, composed of a first spatial axis, denoted x, a second spatial axis, denoted y, and a third spatial axis, denoted z. In the present disclosure, the elements of the imaging spectrometer 100 (and 200 for the second embodiment) are defined with respect to this orthonormal spatial reference frame.

[0052] The geometric center 11 is positioned at the intersection between a first median axis 12 of the entrance slot 10 defined along the first spatial axis x and a second median axis 14 along the second spatial axis y.

[0053] In the present disclosure, by median axis of an object is meant the axis which cuts said object into two equal parts when this object is seen in a plane. For example, this median axis can separate the right half of this object from the left half or the upper part of this object from the lower part, said right half having an area equal to the left part and said upper part having an area equal to said lower part.

[0054] Since the inlet slot 10 is rectangular in shape, the first median axis 12 is perpendicular to the second median axis 14.

[0055] Here, as illustrated in [Fig.2], the height 13 of the inlet slot is defined parallel to the first median axis 12 and the width 15 of the inlet slot is defined parallel to the second median axis 14.

[0056] The inlet slot 10 comprises a longitudinal axis 16. This longitudinal axis is oriented parallel to the height 13 of the inlet slot. In particular, in this example, this longitudinal axis is included in the first median axis 12, it is therefore aligned along the first spatial axis x. Thus, the longitudinal axis 16 coincides with the first median axis 12. This longitudinal axis 16 passes through the geometric center 11 of the inlet slot 10 and intersects perpendicularly the second median axis 14 of the inlet slot 10. It is therefore also perpendicular to the second spatial axis y.

[0057] In this configuration, the height 13 has an elongation direction aligned along the first spatial axis x and the width 15 has an elongation direction defined along the second spatial axis y perpendicular to the first spatial axis x.

[0058] As illustrated, the height 13 (here the value of the height 13) is greater than or equal to the width 15 (i.e. value of the width 15), here in particular greater than the width 15 of the entry slot 10.

[0059] Typically, the entrance slit 10 used in the imaging spectrometer 100 illustrated in [Fig.l] has a height 13 of between 4 pm and 28 mm, i.e. comprising any of the values between 14 pm and 28 mm. In this example, the height of the entrance slit 10 is typically 2.7 mm.

[0060] When the entrance slit 10 is rectangular, the width 15 of the slit is equal to or less than the height 13 of the slit. Typically, the width of the slit is between 4 pm and 1 mm. In practice, if the entrance slit 10 is made of a single opening as shown in [Fig.2], the width of the slit can be between 10 pm and 500 pm, which makes it possible to improve the performance of the imaging spectrometer. However, if the entrance slit is formed by a bundle of optical fibers as shown in [Fig.9], the width of the entrance slit can be between 4 pm (for example in the case of using single-mode optical fibers) and 1 mm (in the case of using optical fibers having a larger core) to obtain better performance in this embodiment. Here, it is for example 50 pm.

[0061] Thus, here, the incident light beam 1 passes through the entrance slit 10 and propagates towards the reflecting optical device 20. In the present disclosure, the term "light beam" means a set of light rays propagating, upstream of the entrance slit 10, in the same direction of propagation. For the sake of simplification, in [Fig.l], a single light ray 2 of the incident light beam 1 is illustrated.

[0062] As illustrated in [Fig.l], the reflective optical device 20 is positioned on the optical path of the incident light beam 1. Here typically, the distance separating the geometric center 11 of the entrance slit 11 and the vertex of the first mirror 21 is 98.83 mm. In the present disclosure, the distances between two objects are expressed relative to the geometric centers of the surfaces.

[0063] In this first embodiment, the reflecting device 20 comprises two mirrors, a first mirror 21 and a second mirror 22. The first mirror 21 and the second mirror 22 are concave mirrors each having an optical axis 23, 24 and a curvature. The reflecting device 20 has a center of curvature 25. Here, in this embodiment, the two mirrors 21 and 22 are concentric and thus have a single center of curvature 25 identical to the two mirrors 21, 22. Furthermore, in this example, the curvatures of the first mirror 21 and the second mirror 22 are identical, for example here they are 97.56 mm. Of course, in a variant of the imaging spectrometer 100, the curvature of the first mirror 21 may differ from the curvature of the second mirror 22 while retaining the concentric arrangement between each mirror 21, 22 and the diffraction grating 30.The first mirror 21 and the second mirror 22 may also have a similar diameter, for example here 50 mm. Of course, in a variant, the first mirror 21 and the second mirror 22 may have different diameters. In this case, the second mirror 22 may have a larger useful surface than the first mirror 21, for example at least 1 mm.

[0064] The reflecting optical device 20 is configured to reflect the incident light beam 1 coming from the entrance slit 10 and direct it towards the diffraction grating 30. Here in particular, the first mirror 21 reflects, here according to the Snell-Descartes laws, the incident light beam 1 towards the diffraction grating 30.

[0065] The diffraction grating 30 is a convex reflection diffraction grating comprising a geometric optical center 31 and an optical axis 32. Here, the optical axis 32 of the diffraction grating 30 is oriented perpendicular to the longitudinal axis 16 of the entrance slit 10. It also comprises a center of curvature 36 positioned along its optical axis 32.

[0066] In this example, the center of curvature 36 of the diffraction grating 30 is equal to 50.42 mm and the diffraction grating 30 has a diameter of 23.5 mm.

[0067] The diffraction grating 30 and the reflecting optical device 20 are arranged concentrically. In practice, this means that at least one of the mirrors among the first mirror 21 and the second mirror 22 is arranged concentrically with the diffraction grating 30. Such an arrangement makes it possible to guarantee the Offner type arrangement and therefore to obtain very good image quality on the imaging detector 40. Typically, in this example, the center of curvature of the first mirror 21, the center of curvature 25 of the second mirror 22, and the center of curvature 36 of the diffraction grating 30 are concentric. They are, in this example, positioned 50.42 mm upstream of the diffraction grating 30. Such a configuration makes it possible to further improve the spectral and spatial resolution of the imaging spectrometer 100.

[0068] Here typically, the distance separating the first mirror 21 or the second mirror 22 and the diffraction grating 30 is 47.14 mm or 45.8 mm after optimization.

[0069] Thus, after having been reflected by the reflecting optical device 20 (here the first mirror 21), the reflected incident light beam 1 propagates towards the diffraction grating 30.

[0070] The diffraction grating 30 may be an etched diffraction grating or a holographic diffraction grating.

[0071] The diffraction grating 30 comprises reflector planes 35 configured to spectrally diffract the incident light beam 1 reflected by the reflecting optical device 20.

[0072] In the present example, the diffraction is continuous for all the wavelengths included in the spectrum of the incident light beam 1. Thus, by spectrally diffracted, it is meant that the diffraction grating 30 is configured to diffract or disperse each light ray 2 of the incident light beam 1 into a packet 5 of diffracted light rays 3, the number of diffracted light rays 3 of which is a function of the wavelengths of the incident light beam, each diffracted light ray 3 of the same packet 5 of diffracted light rays propagating at a wavelength included in the spectrum of the incident light beam following a propagation direction which is a function of the wavelength. In other words, this means that each diffracted light ray 3 from the same parquet has a wavelength which is different from the wavelengths of the other diffracted light rays from the same packet 5. It is also understood that the diffracted light beam is formed by all the diffracted light rays 3 by the diffraction grating 30. Here, in the example of [Fig.l], as only one light ray 2 from the incident light beam 1 is illustrated, the packet 5 of the diffracted light rays 3 illustrates the diffracted light beam 4. In the case where several packets 5 of diffracted light rays 3 are formed by the diffraction grating 30, all the packets 5 correspond to the diffracted light beam 4.

[0073] In practice, the diffraction grating 30 is configured to diffract the incident light beam 1 reflected by the reflecting device 20 according to the grating law known to those skilled in the art. Typically, the diffraction grating 30 is arranged to favor a diffraction order by maximizing the intensity of the diffracted beam according to a diffraction order. The maximized diffraction order may be order 1 in absolute value, that is to say order 1 or -1. In practice, the diffracted light beam 4 at order 1 or -1 has an intensity much higher than the intensity of the diffracted light beams at the other diffraction orders. Thus, the intensity of the diffracted light rays in the other orders is negligible compared to the intensities of the diffracted light rays at the maximized order. For example, the maximized diffraction order is order -1.Thus, the diffraction grating 30 diffracts the incident light beam 1 coming from the reflecting optical device 20 according to the diffraction order equal to - 1. In other words, this means that each incident light ray 2 of the incident light beam is spectrally diffracted by the diffraction grating 30 into a packet 5 of diffracted light rays 3 according to the diffraction order equal to -1. The order -1 makes it possible in particular to obtain a more compact imaging spectrometer.

[0074] The reflecting planes 35 are separated by grooves 33 of the diffraction grating 30. As illustrated in [Fig.2], the grooves 33 are parallel to each other and are defined parallel to the first spatial direction x. Consequently, the grooves 33 are each oriented parallel to the longitudinal axis 16 of the slit 10. Here, typically, the grooves 33 have a density of approximately 770 lines per millimeter (tr / mm).

[0075] The grooves are configured to delimit the reflecting planes 35 of the diffraction grating 30. Each groove 33 is spaced from an adjacent groove 33 by a distance 34 (also noted a), which is similar in this example. In other words, here, all the grooves are spaced by the same distance 34 (i.e. constant distance 34), the diffraction grating 30 is therefore a constant pitch diffraction grating. For example, the distance is 1.3 pm (1 / 770 mm). Of course, as will be illustrated in [Fig.9], the diffraction grating 30 may be a variable pitch diffraction grating for which the distance 34 may vary from one groove 33 to another.

[0076] As an example, [Fig.4] illustrates in a profile view a projection of the diffraction grating 30 in a plane perpendicular to the optical axis 32 of the diffraction grating 30. Indeed, this projection erases, in this representation, the curvature of the diffraction grating 30. As a variant, the reflecting planes 35 are of planar shape. According to a particular aspect, the succession of the different reflecting planes 35 forms saw teeth of the same shape.

[0077] Of course, the set of reflector planes 35 can form other patterns, for example a sinusoidal pattern, a square pattern, etc. In addition, the reflector planes 35 can have a radius of curvature (convex or concave), a rectangular shape, a square shape, etc.

[0078] In [Fig.4] and in [Fig.l], a light ray 2 of the incident light beam 1 arrives on the diffraction grating 30 with an incidence inclined in the plane (yz). Thus, each incident light ray 2 reflected by the first mirror 21 arriving on the diffraction grating 30 has an angle of incidence alpha defined in the plane (yz). The angle of incidence Alpha is the angle between the incident beam and a normal to the curved surface of the diffraction grating 30 at the point of contact of this beam on this surface of the diffraction grating 30. For example, in the plane defined by the second spatial axis and the third spatial axis, the angle of incidence alpha associated with this incident light ray 2 is here 22.7 degrees. Similarly, this incident light ray 2 arrives on the diffraction grating with an incidence inclined in the plane (xz) following an azimuthal angle of incidence (noted sigma in [Fig.7]).For example, in this example, the azimuthal angle of incidence sigma is 34.7 degrees. These incidence angle values allow for overall optimization of the system, particularly favoring the geometric compactness of the system.

[0079] In the plane (yz), this incident light ray 2 is diffracted to form a packet 5 of diffracted light rays 3, each ray 3 of the same packet 5 being diffracted, depending on its wavelength, according to an angle beta, noted here beta1, beta2, etc.

[0080] Here, in particular, the angles of incidence and diffraction are linked by the well-known formula of the diffraction grating sin (alpha) + sin (beta) = m lambda / a, with m the order of diffraction equal to -1, a corresponding to the distance or spacing between the adjacent grooves (corresponding to the distance 34) and lambda the wavelength of the diffracted light beam 3.

[0081] Thus, for a given angle of incidence alpha, the angle beta of one of the diffracted light rays 3 of the diffracted light beam 4 depends on the distance 34 (also denoted a) associated with the reflector plane 35 on which the incident light ray 2 and the wavelength of the corresponding diffracted light ray 3 diffract. Thus, each wavelength of the spectrum of the incident light beam 1 is diffracted at a beta angle, spectrally dispersing the different wavelengths of the incident light beam 1.

[0082] It is understood that diffraction applies in a similar manner in the (xz) plane with alpha this time corresponding to the azimuthal angle of incidence.

[0083] As illustrated in [Fig.2] and [Fig.3], the geometric center 11 of the entrance slit 10 and the geometric optical center 31 of the diffraction grating 30 are connected by a straight line 60, which is a fictitious straight line in the imaging spectrometer 100. This straight line 60 delimits a distance d separating the geometric center 11 of the entrance slit 10 from the geometric optical center 31 of the diffraction grating 30. In practice, the geometric center 11 of the entrance slit 10 is spaced from the optical center 31 of the diffraction grating by a distance d of between 15 mm and 50 mm. In this example, this distance d is 61.6 mm in space (xyz) and 31.14 mm when projected into the plane (xy).

[0084] In the present disclosure, the straight line 60 forms an angle 61 in the plane (xy) with the longitudinal axis 16 of the entrance slit 10. This angle 61 is in the present disclosure greater than 0 degrees and less than 45 degrees or greater than 45 degrees and less than 90 degrees. Advantageously, the angle may be between 45 degrees and 70 degrees (45 degrees excluded) and preferably between 50 degrees and 70 degrees. Such an arrangement makes it possible to obtain an imaging spectrometer with a large numerical aperture (NA) (here for example 0.22) while retaining the optical quality of this imaging spectrometer and improving the compactness of the imaging spectrometer. Furthermore, as will be described below, the spectrum obtained by such an imaging spectrometer has a wide spatial extent and is highly resolved over the slit height with excellent spatial resolution over the entire height of the imaged entrance slit on the imaging detector.

[0085] It is thus understood that in the plane (xy), the geometric optical center 31 of the diffraction grating 30 has a spatial position defined relative to the geometric center 11 of the slit 10 and the angle θ formed between the longitudinal axis 16 and the straight line 60. Thus, it is possible to position the diffraction grating 30 relative to the entrance slit 10 using polar coordinates having as their origin the geometric center 11 of the entrance slit 10 and defined by the distance d and the angle θ.

[0086] In the present example, very good imaging qualities are obtained with an angle 61 of 56.68 degrees when the distance d is 31.14 mm.

[0087] After being diffracted by the diffraction grating 30, the diffracted light beam 4 propagates again towards the reflecting optical device 20, here the second mirror 22 of the reflecting optical device 20.

[0088] In the present disclosure, the reflecting device 20 is also configured to reflect the diffracted light beam 4 spectrally by the diffraction grating 30 and direct it towards the imaging detector 40. In practice, here the second mirror 22 reflects the diffracted light beam 4 according to the Snell-Descartes law and directs it towards the imaging detector 40. The diffracted light beam reflected by the second mirror 22 thus propagates after its reflection towards the imaging detector 40.

[0089] In particular here, the reflective optical device 20 is configured to correct the field aberrations including the image distortion on the imaging detector 40 thanks to the concentric arrangement between the two mirrors 21 and 22.

[0090] The imaging detector 40 comprises a recording surface 41, generally of planar shape, arranged to record a spectrum of the diffracted light beam reflected by the reflecting optical device 20 (here the second mirror 22).

[0091] The recording surface 41 has a first spatial dimension 42, corresponding here to a height of the recording surface 41, and a second spatial dimension 43 perpendicular to the first spatial dimension 42 and corresponding to a width of the recording surface 4L.

[0092] As illustrated in [Fig.l] and in [Fig.3], the first spatial dimension 42 of the recording surface 41 is oriented parallel to the longitudinal axis 16. In order to be able to acquire all the light rays 2 of the incident beam 1 passing through the entrance slit 10, the first spatial dimension 42 of the recording surface is adapted to the height 13 (here the value of the height 13) of the entrance slit 10. In other words, this means that the first spatial dimension 42 of the recording surface 41 is arranged to capture all the incident light rays 2 passing through the entrance slit 10. In practice, the value of the first spatial dimension 42 of the recording surface 41 is greater than the value of the height 13 of the entrance slit 10. Such a characteristic depends on the magnification of the system, which is preferably equal to or greater than 1 (here 1 for example).

[0093] The second spatial dimension 43 of the recording surface is adapted to the spectral diffraction of the incident light beam produced by the diffraction grating 30. In particular here, the second spatial dimension 43 of the recording surface 41 is adapted to the extent of the spectrum of the packets 5 of the light beam diffracted 4 by the diffraction grating 30 and reflected by the reflecting optical device 30.

[0094] It is understood here that each diffracted ray 3 of the same packet 5 has a spatial position distinct from the other diffracted rays of the same packet 5 on the recording surface 41 aligned along the second spatial dimension 43 of the recording surface 41.

[0095] It is also understood that the dimensions of the recording surface 41 depend in particular on the entrance slit 10 and on the diffraction order of the diffracted light beam 4 by the diffraction grating 30. Here in particular, the first spatial dimension 42 of the recording surface 41 depends on the height 13 of the entrance slit 10 and the second spatial dimension 43 of the recording surface 41 depends on the diffraction grating 30, in particular on the diffraction order of the diffraction grating 30.

[0096] The recording surface 41 of the imaging detector 40 has a geometric center 44 and a normal axis 45 to said recording surface 41 passing through the geometric center 44 of the recording surface 41. As illustrated in [Fig.l] or in [Fig.6], the normal axis 45 (or normal 45) of the recording surface 41 is parallel to the optical axis 32 of the diffraction grating 30. It is therefore also oriented perpendicular to the longitudinal axis 16 of the entrance slit 10.

[0097] In this example, the imaging spectrometer 100 optionally comprises a filter 50 positioned on the optical path of the diffracted light beam reflected by the reflecting device 20 (here the second mirror 22). The filter 50 is positioned upstream of the imaging detector 40 in particular with respect to a surface of the imaging detector, here for example 6 mm before the surface of the imaging detector 40. In other words, this means that the filter 50 is positioned between the imaging detector 40 and the reflecting optical device 20 (here corresponding to the second mirror 22).

[0098] The filter 50 is a filter operating in transmission configured to filter the reflected diffracted light beam according to the diffraction orders of said reflected diffracted light beam. In particular, here, the filter 50 is configured to select a single diffraction order of the diffracted light beam. In practice, the selected diffraction order corresponds to the diffraction order associated with the diffracted light beam maximized by the diffraction grating 30, i.e. the one that is diffracted by the diffraction grating 30 with the greatest intensity. Thus, the filter 50 is arranged to filter (i.e. cut, stop) the diffraction orders of the diffracted light beam different from the selected diffraction order. If in the application the useful order of the diffraction grating 30 is order -1, the filter 50 filters orders -2, -3, -4 ...If the useful order of the diffraction grating 30 is order +1, then the filter 50 filters orders +2, +3, +4, etc. Furthermore, the filter 50 also makes it possible to filter out the parasitic rays that can be created at each reflection of the light beam on an element of the imaging spectrometer 100, here for example at the level of the slit, or at the level of the mirrors 21, 22 of the reflecting device 20.

[0099] Such an arrangement makes it possible to transmit only the diffracted light rays 3 to the imaging detector 40 according to the selected order (here order -1), the other orders and other parasitic rays (to a lesser extent) being deflected, reflected or absorbed by the filter 50. This makes it possible to improve the quality of the spectrum recorded by the imaging detector 40.

[0100] As illustrated in [Fig.6], the filter 50 is in the form of a blade comprising two parallel faces 51, 52, a first face 51 oriented towards the reflecting optical device 20 (here corresponding to the second mirror 22) and a second face 52, opposite the first face 51 and oriented towards the imaging detector 40. Here, the two faces 51, 52 are spaced apart by a distance d2 preferably less than 5 mm. Preferably, this distance d2 is chosen to be as small as possible while still being able to perform the optical function of this part. For example, here, the distance is equal to 0.5 mm.

[0101] The filter 50 has a geometric center 53, here positioned in a plane 54 of the filter 50 parallel to the first face 51 and the second face 52 and positioned equidistant from the first face 51 and the second face 52. The plane 54 has a first dimension 59 and a second dimension perpendicular to the first dimension of the plane 54 and oriented parallel to the grooves 33 of the diffraction grating 30 (i.e. also oriented parallel to the longitudinal axis 16 of the entrance slit 10). It is understood that the first dimension of the filter 59 is oriented parallel to the first spatial dimension 42 of the recording surface 4L

[0102] In this example, the filter 50 is configured to completely cover the imaging detector. In other words, the plane 54 of the filter 50 has an area greater than the area of the recording surface 4L. For example, in this example, the first dimension of the filter 59 is 16 mm while the second dimension is 34 mm.

[0103] The filter 50 also comprises a central axis 55, normal to the first face 51, to the plane 54 and to the second face 52 and passing through the geometric center 53 of the filter 50.

[0104] As illustrated in [Fig. 1], the filter 50 is inclined relative to the recording surface 41 of the imaging detector 40 to limit the capture of parasitic reflections. Here, it can be seen that the filter 50 is inclined relative to the recording surface 41 by a rotation about an axis y2 by a first angle 56. As illustrated in the figure around an axis y2 positioned at the geometric center 53 of the plane 54 oriented perpendicular to the normal axis 45 of the recording surface and parallel to the axis y. This axis y2, originating from the orthonormal reference frame x2, y2 having as its origin the geometric center of the plane 54, is oriented parallel to the axis on which the second spatial dimension 43 of the recording surface 4L is aligned. The first angle of inclination 56 is between 5 degrees and 45 degrees, preferably between 10 degrees and 45 degrees. Here, typically, the tilt angle 56 is 32 degrees to achieve better performance. The plane 54 is therefore also inclined relative to an axis 62 (oriented parallel to the longitudinal axis 16 of the entrance slot 10 and oriented parallel to the first dimension 42 of the recording surface 41) at an angle of inclination equal to the first angle of inclination described above.

[0105] Thus, the light beam diffracted by the diffraction grating is reflected on the second mirror 22 of the reflecting device 20. The latter is configured to orient this reflected diffracted light beam towards the imaging detector 40. Here, as the filter 50 is positioned on the optical path of the diffracted light beam reflected by the reflecting optical device 20 (here the second mirror 22), the diffracted light beam is filtered by the filter (i.e. only the light rays diffracted to the useful order (here the order -1) are retained). The diffracted light beam filtered by the filter propagates in the filter 50 and then reaches the imaging detector 40.

[0106] In the present disclosure, the imaging spectrometer is configured to form an image of the entrance slit 10 on the recording surface 41 of the imaging detector 40.

[0107] In this example, the imaging detector 41, in particular the recording surface 41, has a size of 2048 pixels (along the second spatial dimension 43) x 92 pixels (along the first spatial dimension 42) with square pixels of size 14 μm. The imaging detector 40 is read by charge summation on the 192 pixels of the slit height to produce a spectrum of the light from the entrance slit 10. Thus, in practice, the imaging detector 40 is configured to collect and sum the values of the pixels of the same column of pixels aligned along the first spatial dimension 42 of the imaging spectrum 40. As a result, a line of the recorded spectrum can be measured by summation of the intensities detected on the pixels of the same column of pixels.

[0108] [Fig.5] illustrates an example of spectrum 101 of the light beam diffracted by the diffraction grating 30 and reflected by the reflecting optical device obtained using the imaging detector 41 described above. The spectrum 101 presents on the abscissa the wavelengths (denoted X) of the spectrum and on the ordinate a relative quantity the height of the entrance slit 10 given here without unit.

[0109] The illustrated spectrum 101 is defined between the wavelengths (X) of 190 nm to 900 nm following a spatial extent 102 of between 28 mm and 30 mm. Here, in particular, the wavelengths of the spectrum are spread over a total of 28 mm, defining the spatial extent 102 of the spectrum. Thus, this means in this example that the selected diffraction order of the diffracted light beam is spread over 28 mm. Such a spatial extent of the spectrum underlines a very good spectral resolution since the wavelengths are sufficiently spaced from each other, which improves readability. of the 101 spectrum. Thus, the imaging spectrometer has a very high resolving power.

[0110] In this example, the acquired spectrum 101 comprises several lines 103, each line 103 being associated with a wavelength of the diffracted light beam 3. The points of the spectrum aligned along the same axis parallel to the abscissa axis are associated with the same incident light ray 1 or corresponding to the same source point on the entrance slit 10 but spectrally diffracted. As illustrated in [Fig.5], the lines 103 have a low spread and are close to a Dirac shape, which underlines the very good quality of the imaging of the imaging spectrometer 100.

[0111] As explained above, each line 103 has an amplitude 104 along the ordinate axis which depends on the height 13 of the entrance slit 10 and / or in the case of [Fig.9] which also depends on the spatial position of the incident light rays 2 on the height of the entrance slit. Indeed, in the case of the use of optical fibers to form the entrance slit, each optical fiber can transmit a light flux having an intensity and / or a propagation which depends on the optical fiber used. The amplitude 104 of each line is approximately equal, highlighting the presence of little optical aberration introduced by the imaging spectrometer 100.

[0112] The imaging spectrometer 100 may also optionally comprise means 70 for adjusting the position of the entrance slit 10. Thus, in this case, it is possible to move the slit along the longitudinal axis 16, which modifies the angle formed between the elongation axis 16 and the straight line 60. It is also possible in combination or as an alternative to move the entrance slit 10 along an axis transverse to the elongation axis 16. In practice, these adjustment means 70 are embodied, for example, by a support carrying the entrance slit 10 and movable along at least one spatial direction, here movable at least along the first spatial dimension x and along the second spatial direction y. Such an arrangement makes it possible to carry out an optical adjustment to compensate for positioning precisions and mechanical machining.

[0113] The imaging spectrometer 100 may also optionally comprise means 72 for tilting the detector along at least one spatial direction, preferably at least one axis oriented parallel to the first spatial direction 42 of the imaging detector in order to optimize the resolution between low and high wavelengths. In a variant, the tilting means 72 are also arranged to tilt the imaging detector 40 along an axis oriented parallel to the second spatial direction of the imaging detector in order to optimize and compensate for a lack of orthogonality of the plane of the slit with the axis of the system.

[0114] A second embodiment of an imaging spectrometer 200 according to the present disclosure will be described with the aid of [Fig.7], [Fig.8] and [Fig.9]. Only the differences with the first embodiment will be described.

[0115] Thus, the imaging spectrometer 200 illustrated in [Fig.5] comprises an entrance slit 80, a reflective optical device element 90, a diffraction grating 110, a filter 50 and an imaging detector 40.

[0116] The input slit 80 used in this second embodiment is that illustrated in [Fig.9]. Of course, this input slit 80 can also be used in the imaging spectrometer 100 described above. Here, the input slit 80 used in the imaging spectrometer 200 is formed by a plurality of optical fibers 87. The optical fibers used may all be similar or may be different. Furthermore, the optical fibers 87 used may comprise single-mode fibers, multi-mode fibers, graded-index fibers, etc. Here in particular, the input slit 10 is formed by twelve optical fibers 87 having a core of 9 μm in diameter.

[0117] Each optical fiber 87 has a free end (here represented by their diameter) delimiting an opening through which the incident light beam 1 exits the fiber and propagates in the direction of the reflecting optical device 20.

[0118] As illustrated in [Fig.9], the optical fibers 87 are aligned successively one behind the other along a longitudinal axis 86, corresponding to the longitudinal axis of the input slot 80.

[0119] Here, the optical fibers 87 each have an optical axis 88 which are here perpendicular to the longitudinal axis 86 of the entry slot 80. Thus, the optical axes 88 of the optical fibers 87 are parallel to each other. Here, these optical axes are arranged to extend along the second median axis 94 of the entry slot 90.

[0120] As the optical fibers have a similar diameter, the optical fibers 87 are fictitiously inscribed in a geometric shape 89, here a rectangle 89. From the geometric shape formed by the alignment of the optical fibers 87, it is possible to define a geometric center 81 of the entry slot 80, corresponding to the geometric center 81 of the rectangle 89 formed by the alignment of the optical fibers 87.

[0121] In a similar manner to the first embodiment, the geometric center 81 is formed at an intersection between a median plane axis 82 of the inlet slot 80 and a second median axis 84 of the inlet slot 80. The inlet slot also has a height 83 oriented parallel to the first median axis 82 and a width 85 oriented parallel to the second median plane 84. As previously, the value of the height 83 of the slot 80 is greater than or equal to the value of the width 85 of the slot, here in particular greater than the value of the width 85.

[0122] The first median axis 82 is perpendicular to the second median axis 84. The longitudinal axis 86 is aligned with the first median plane 82, it therefore passes through the geometric center 81 and is perpendicular to the second median axis 84.

[0123] Similar to the first embodiment, the diffraction grating 110 of the imaging spectrometer 200 has an arrangement similar to that described in the first embodiment except that the diffraction grating 110 of the imaging spectrometer has a variable pitch. The grooves 113 of the diffraction grating 110 are spaced apart by non-constant distances, denoted a1, a2, a3, etc. Such a configuration makes it possible to improve the optical performance of the imaging spectrometer 200 compared to an imaging spectrometer using a constant pitch diffraction grating. In particular, such a configuration makes it possible to limit field aberrations, such as distortion, thus making it possible to increase the spectral domain and the slit height. The scope of use of the imaging spectrometer 200 is therefore improved.

[0124] As in the first embodiment, the diffraction grating 110 is a convex reflection grating having an optical axis 112 passing through a geometric center 111 of the diffraction grating 110 (also corresponding to the optical center of the diffraction grating 110). It therefore has a diameter of 23.5 mm and a radius of curvature of 50.42 mm.

[0125] As described above, the geometric center 81 of the entrance slit 80 and the optical center 111 of the diffraction grating 110 are connected by a straight line 120, which is a fictitious straight line in the imaging spectrometer 200. This straight line 120 delimits a distance d separating the geometric center 81 of the entrance slit 80 from the optical center 111 of the diffraction grating 110.

[0126] The straight line 120 forms an angle 121 in the plane (xy) with the longitudinal axis 16 of the entrance slit 80. This angle 120 is greater than 0 degrees and less than 45 degrees or greater than 45 degrees and less than 90 degrees. Advantageously, the spectral characteristics and the resolution along the slit height are achieved for an angle 120 between 45 degrees (excluded) and 70 degrees, preferably between 50 degrees and 60 degrees.

[0127] This geometric center 111 has a spatial position in the imaging spectrometer 200 defined relative to the geometric center 81 of the entrance slit 80, here typically using polar coordinates. Thus, the optical center 111 of the diffraction grating 110 has a spatial position defined relative to the geometric center 81 of the slit 80 and the angle θ formed between the longitudinal axis 86 and the straight line 120.

[0128] In the imaging spectrometer 200, the reflective optical device 90 is positioned in a concentric configuration with the diffraction grating 110. For Consequently, the center of curvature 93 of the reflecting optical device 90 and the center of curvature 123 of the diffraction grating 110 are superimposed, they are merged.

[0129] In this embodiment, the reflective optical device 90 is made solely of a concave mirror 90. In this example, the mirror 90 has a radius of curvature of 97.56 mm and a size of 104 mm (along a direction parallel to the x-axis) by 70 mm (along a direction parallel to the y-axis).

[0130] Therefore, in this embodiment, fewer optical components are used, which facilitates the adjustments of the spectrometer 200 and limits the costs of such a spectrometer 200.

[0131] Here typically, the distance separating the geometric center 81 of the entrance slit 80 from the mirror 90 is 98.93 mm, the distance separating the mirror 90 from the diffraction grating is 45.80 mm and the distance separating the filter 50 from the mirror 90 is 92.06 mm. Thus, the total size of the imaging spectrometer is 98.03 mm.

[0132] The reflecting optical device 90 (here corresponding to the mirror 90) has an optical center 91 and an optical axis 92 passing through said optical center 91. Here, the optical axis 92 of the reflecting device 90 is oriented perpendicular to the longitudinal axis 86 of the entrance slit 80 (and also to the grooves 113 of the diffraction grating 110).

[0133] The optical axis 92 of the reflecting device 90 is parallel to the optical axis 112 of the diffraction grating 110, in particular here the optical axis 92 of the reflecting device 90 is aligned with the optical axis 112 of the diffraction grating 110.

[0134] Thus, in this example, the mirror 90 is arranged to receive the incident light beam 1 coming from the entrance slit 80 and reflect it towards the diffraction grating 110. The mirror 90 is also arranged to receive the diffracted light beam 4 by the diffraction grating 110 and reflect it towards the imaging detector 40.

[0135] The present invention is in no way limited to the embodiments described and shown, but those skilled in the art will be able to provide any variant in accordance with the invention.

[0136] In a non-limiting manner, the entrance slit 80 illustrated in [Fig.9] can be used in the imaging spectrometer 100 according to the first embodiment and similarly the entrance slit 10 illustrated in [Fig.3] can be used in the imaging spectrometer 200.

Claims

1. Claims An Offner-type imaging spectrometer (100, 200) comprising: - an input slit (10) configured to transmit an incident light beam (1) comprising wavelengths, said input slit (10, 80) having a geometric center (11, 81), a height (13, 83) defined along an axis (12, 82) and a width (15, 85), the value of the height (13, 83) of the input slit (10, 80) being greater than or equal to the value of the width (15, 85) of the input slit (10, 80), the input slit (10, 80) also having a longitudinal axis (16, 86) passing through the geometric center (11, 81) and being oriented parallel to the axis defining the height;- a reflective optical device (20, 90) and a reflective convex diffraction grating (30, 110) arranged in a concentric configuration, i) the reflective optical device (20, 90) having a center of curvature (25, 93) and being configured to reflect the incident light beam (1) and direct it towards the diffraction grating (30, 110); ii) the diffraction grating (30, 110) having a geometric optical center (31, 111) on the surface, the diffraction grating (30, 110) comprising grooves (33, 113) oriented parallel to the longitudinal axis (16, 86) of the entrance slit (10, 80); said diffraction grating (30, 110) being arranged to spectrally diffract the incident light beam (1) reflected on the reflecting optical device (20, 90), iii) the reflective optical device (20, 90) being configured to reflect the diffracted light beam (4) spectrally by the diffraction grating (30, 110); - an imaging detector (40) configured to record the diffracted light beam (4) by said diffraction grating (30, 110) and reflected by the reflecting optical device (20, 90), the imaging spectrometer (100, 200) being configured to form an image of the entrance slit (10) on the imaging detector (40), characterized in that a straight line (60, 120) joining the geometric optical center (31, 111) of the diffraction grating (30, 110) and the geometric center (11, 81) of the entrance slit (10, 80) forms an angle (61, 121) with the longitudinal axis (16, 86) of the entrance slit (10, 80), said angle (61, 121) being greater than 0 degrees and less than 45 degrees or greater than 45 degrees and less than 90 degrees.

2. An imaging spectrometer (100, 200) according to claim 1, wherein said angle (61, 121) is between 50 degrees and 70 degrees.

3. An imaging spectrometer (100, 200) according to claim 1, wherein said angle (61, 121) is between 50 degrees and 60 degrees.

4. An imaging spectrometer (100, 200) according to claim 1, wherein said angle (61, 121) is between 15 degrees and 40 degrees.

5. Imaging spectrometer (100, 200) according to any one of claims 1 to 4, wherein the diffraction grating (30, 110) is arranged to diffract the incident light beam (1) reflected by said reflecting device (20, 90) according to a diffraction order equal to 1 in absolute value and the imaging detector (40) is arranged to record the diffracted light beam (4) in the diffraction order equal to 1 in absolute value.

6. An imaging spectrometer (100, 200) according to any one of claims 1 to 5, wherein the imaging detector (40) comprises a recording surface (41) having a first spatial dimension (42) and a second spatial dimension (43) perpendicular to the first spatial dimension (42), said first spatial dimension (42) of the recording surface (41) being oriented parallel to the longitudinal axis (16, 86) and being adapted to the height (13, 83) of the entrance slit (10).

7. An imaging spectrometer (100, 200) according to any one of claims 1 to 6, wherein the imaging detector (40) is configured to record a spectrum of the diffracted light beam (4) defined between wavelengths of 190 nm to 900 nm and having a spatial extent of between 28 mm and 30 mm.

8. An imaging spectrometer (100, 200) according to any one of claims 1 to 7, further comprising a filter (50) operating in transmission disposed upstream of the imaging detector (40) and configured to select a single diffraction order of the diffracted light beam.

9. An imaging spectrometer (100, 200) according to claim 8, wherein the filter (50) has a central axis (55) normal to a plane (54) of the filter (50) and passing through a geometric center (53) of the filter (50), said central axis (55) of the filter (50) being inclined relative to a normal to a plane of the imaging detector (40) at an angle of inclination.

10. An imaging spectrometer (100, 200) according to claim 9, wherein said tilt angle is between 5 degrees and 45 degrees.

11. An imaging spectrometer (200) according to any one of claims 1 to 10, wherein the reflective optical device (90) comprises a concave mirror (90) having an optical axis (92) aligned with an optical axis (112) of the diffraction grating (110).

12. An imaging spectrometer (100) according to any one of claims 1 to 10, wherein the reflecting optical device (20) comprises two concave mirrors (21, 22), respectively called first mirror (21) and second mirror (22), the first mirror (21) being configured to reflect the incident light beam (1) towards the diffraction grating (30) and the second mirror (22) being configured to reflect the diffracted light beam (4) towards the imaging detector (40), at least one of the two mirrors (21, 22) among the first mirror (21) and the second mirror (22) being arranged concentrically with the diffraction grating (30).

13. An imaging spectrometer (100) according to claim 12, wherein the first mirror (21) and the second mirror (22) have different radii of curvature.

14. An imaging spectrometer (100, 200) according to any one of claims 1 to 13, wherein the entrance slit (10) is formed by a rectangular opening (17) or the entrance slit (80) is formed by a plurality of optical fibers (87) arranged along the longitudinal axis (86) of the entrance slit (80) and each having an optical axis (88), the optical axes (88) of said optical fibers (87) being parallel to each other.

15. An imaging spectrometer (100,200) according to any one of claims 1 to 14, wherein the diffraction grating (30,110) is formed of non-equidistant grooves (113).

Images